1. Field of the Invention
The present invention pertains to a method and system for measuring the vapor pressure of a liquid, in particular a mixture of hydrocarbon liquids, wherein the liquid is sampled by flowing it through an eductor and the total pressure at the vapor or suction port of the eductor is measured as a function of the pressure differential of the liquid across the eductor at a given temperature.
2. Background
Liquids flowing through pipelines and process equipment, for example, often require the determination of the vapor pressure of the liquid and/or the approximate composition of the liquid so that operating conditions and/or liquid compositions may be controlled to avoid collection of volatile vapors or other operating problems. For example, liquid hydrocarbons being pumped through transmission pipelines and process equipment must be controlled to avoid the collection or dissemination of volatile vapors. Many liquid hydrocarbon flowstreams contain various liquid compositions and the exact or even approximate composition of the hydrocarbon mixture is difficult to determine or monitor under actual pipeline or process equipment operating conditions.
In the transport of crude oil through transmission pipelines from a gathering point to a refining or transfer point, the working pressures must be controlled or the inclusion of components such as natural gas liquids in the crude oil must be controlled to avoid dangerously high levels of volatile vapors. For example, if crude oil containing natural gas liquids (NGL) is being pumped through a pipeline system, the pumping pressures must be controlled or the quantity of NGL in the crude oil must be controlled to avoid a pressure condition wherein the more volatile liquids evaporate and result in the leakage of vapors from pumping equipment seals and other likely points of dangerous vapor accumulations.
One way to determine the vapor pressure of a particular liquid composition is by flowing the composition through a nozzle or eductor to lower the total pressure in the fluid flow stream to the point which is limited by the vapor pressure of the liquid. This condition can be rather easily reached for a liquid composition which is "pure" or does not include any dissolved gases. However, in the aforementioned situation involving the pumping of crude oil and other hydrocarbon liquids, for example, certain volatile dissolved gases and a variety of hydrocarbon compositions of different vapor pressure characteristics may be present in the liquid flow stream and an accurate reading of the vapor pressure by merely flowing the liquid through an eductor at a particular total differential pressure may not provide the actual vapor pressure of the composition being measured.
Moreover, it is important to determine, for a given composition of liquid having a particular quantity of dissolved gas, the pressure at which such dissolved gas begins to come out of solution to avoid the collection or dissolution of various vapors or gases. In regard to the aforementioned situations, the present invention has been developed with a view to providing a method and system for determining the actual pressure at which dissolved gases in a particular liquid composition will come out of solution, for determining the actual vapor pressure of a particular liquid composition and for determining the general composition of a liquid, particularly mixtures of liquid hydrocarbons being transported through pipelines and process equipment.